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Appendix H Soils

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Mass Wasting

Slope Stability Analysis œ Shallow Transnational Slides (Shallow Rapid Slides)

1. Planar and Convex Slopes

The analysis considered average present conditions after fire (Burned Condition) and prior to fire (Unburned Condition). The results of the analysis - factor of safety (FS) and probability of failure (Pf) - are as follows:

Table H-1. Slope Stability of Planar and Convex Slopes

Slope (%)

Unburned Condition (forested)

Burned Condition (denuded)

FS Pf (%) FS Pf (%)

65 – 70 1.25 0 1.26 0

70 - 75 1.18 0.1 1.20 0.1

75 – 80 1.13 7 1.08 20

80 – 85 1.08 24 1.03 42

85 – 90 1.03 42 0.99 60

2. Concave and Draw Slopes

The analysis considered average present conditions after fire (Burned Condition) and prior to fire - (Unburned Condition). The results of the analysis - factor of safety (FS) and probability of failure (Pf) - are as follows:

Table H-2. Slope Stability of Concave and Draw Slopes

Slope (%)

Unburned Condition (forested)

Burned Condition (denuded)

FS Pf (%) FS Pf (%)

50 – 55 1.29 0 1.18 0.7

55 – 60 1.19 0.2 1.09 10

60 – 65 1.11 5.5 1.01 41

65 – 70 1.04 26 0.95 80

70 – 75 0.98 60 0.90 97

Notes:

• The above analyses consider average conditions of the topography and geomorphology. Micro-site conditions (i.e., spring areas, wetlands, areas with internal piping, talus areas, shallow and exposed bedrock areas, etc.) may exist that could differ from these averages.

• Factor of Safety (FS) is the ratio between resisting and driving forces acting on a solid body. The Probability of Failure (Pf) is the relative frequency of factors of safety less than one.

• Slopes over 90 percent may in fact have higher factors of safety (be safer) than indicated above. The depth of soil is less than assumed in the analyses and bedrock outcrops are frequently encountered.

• High ground water conditions (a major storm event) were assumed in the analyses.

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• The determination about acceptable risk related to harvesting steep slopes should be made based on consideration of probability of failure (Pf) and consequences of such event occurring. Unacceptable consequences for this project would be initiation of debris torrent, deposition of landslide material onto adjacent private land, and deposition of land slide material into perennial streams. For the substantial, adverse consequences situations, harvesting of trees on slopes with probability of failure (Pf) greater than 20 percent should not be permitted without more detailed, site-specifi c analysis, i.e., convex and planar slopes over 85 percent, and concave slopes greater than 75 percent. For low consequence slopes, an acceptable probability of failure may be as high as 33 percent, or 1 in 3.

The above analysis indicates the following:

1. Planar and convex slopes can be harvested with minimum risk of triggering landslides.

2. Concave slopes (draws, hollows) have higher, but acceptable risk of slope instabilities.

3. Depending on the level of acceptable risk, areas that exceed these thresholds should not be harvested. In draws, the risk of mass wasting will exceed 10 percent for slopes steeper than 75 percent.

4. When areas are found during field preparations of the units for harvest where obvious instabilities exist and the consequences of failure are high, these micro-sites should be excluded from harvesting.

Debris Torrent Analysis œ Mass Movement in Steep Stream Channels

The analysis is based on an analytical method outlined in …. (Ref.) It takes into account the size of streambed material, stream gradient, and quantity of flow within the channels. The analysis considers pre-fire, and short-term and long-term post-fire changes in stream flows. Three states of the channel are contemplated: stable streambed, transport of surface streambed material, and torrent (massive movement of the entire stream bed strata).

Table H-3. Mass Movement in Stream Channels, Pre- and Post-Fire Channel Gradient

Pre-fire Condition

Post-fire Condition (< 5 years)

Post-fire Condition (> 5 years)

10% stable transport stable

20% stable transport transport

30% stable/transport transport/torrent transport

40% transport torrent transport

Notes:

• The above analyses consider average conditions of the topography and geomorphology. Micro-site conditions may exist that could differ from these averages.

• Assumptions made in the above analysis: average substrate material size, D50 is 3”, post-fire channel flow increases are approximately 150 percent.

Slope Stability Analysis œ Road Fill Failures

The analysis considered average conditions of a road stream crossing placed on varying stream gradient. The results of the analysis - fill height, fill volume, and factor of safety (FS) - are as follows:

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Table H-4. Conditions of Road Stream Crossings

Slope (%)

Fill Height (ft)

Fill Volume (ydý)

Factor of Safety (FS)

10 5 150 1.67

20 8 300 1.42

30 9 450 1.21

40 11 700 1.12

50 13 1,000 0.99

Notes:

• The above analyses consider average conditions of the topography and geomorphology. Micro-site conditions (fill material, condition of drainage structure) may exist that could differ from these averages.

• Factor of Safety (FS) is the ratio between resisting and driving forces acting on a solid body. A FS greater than 1.2 is considered safe for a non-critical road structure. The probability of failure is less than 10 percent. A Factor of Safety less than 1.1 is unacceptable. The probability of failure is high (25 percent or greater).

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